Add release notes

README.md

@@ -337,6 +337,17 @@ See the corresponding directories for information on how to use the provided con
 
 # [Changelog](https://github.com/openmm/openmmforcefields/releases)
 
+## 0.13.0 Temporarily remove GAFFTemplateGenerator
+
+This release temporarily removes GAFFTemplateGenerator because of packaging incompatibilities with
+AmberTools 23. This functionality is planned to be re-introduced in 0.14.0.
+
+This release is expected to work with Python 3.10-3.12.
+
+Other changes include
+* The default force field of `SystemGenerator` was updated from `openff-1.0.0` (code name Parsley) to
+  `openff-2.0.0` (code name Sage).
+
 ## 0.12.0 Updates for espaloma and support a offxml string in SystemGenerator
 See our [0.12.0 release page](https://github.com/openmm/openmmforcefields/releases/tag/0.12.0) for more details.